Solar cells must undergo numerous manufacturing steps during fabrication. Great care is usually required at each of these steps to ensure that the solar cells meet their target specifications (performance metrics), such specifications including, amongst others, conversion efficiency, open circuit voltage, short circuit current density, and fill factor.
The field of concentrated photovoltaics (CPV), or concentrator photovoltaics, where sunlight is focused (concentrated) on solar cells, has been the focus of much research, development, and commercial activities recently. One of the goals of CPV is to generate more power out of smaller size solar cells. This requires high efficiency solar cells having a small area. Exemplary dimensions of solar cells for CPV can range from, for example, 15 mm×15 mm to 1 mm×1 mm or less.
Typically, multi-junction solar cells for CPV are epitaxially grown on a same semiconductor wafer and are subsequently separated from each other by sawing the wafer along pre-determined sawing lanes or cutting lanes. One of the disadvantages of sawing the semiconductor wafer is that sawing will typically create defects along the saw cut, at the perimeter of the solar cells. Photo-generated carriers, namely, electrons and holes, which are within a diffuse length of such defects, can diffuse to the defects and become trapped, or recombine, at the defects, thereby degrading the performance metrics of the solar cells. Chemical and surface passivation, such as wet etch or SiN plasma-enhanced chemical vapor deposition, can be used to remove or passivate these defects; however, it is likely that a portion of the defects will remain. Further, the chemicals used in some passivation processes are environmentally-unfriendly and their disposal is costly.
The decrease in the performance metrics of the solar cell and can be particularly felt in small solar cell made of high quality material, which provides long diffusion lengths for the photo-generated carriers. Such long diffusion lengths allow a considerable fraction of the photo-generated carriers to reach the perimeter of the solar cell to recombine (or become trapped) at a defect created by the sawing process. In the past, high efficiency solar cells have been used primarily in space applications, e.g., to power satellites. In such applications, the solar cells have a large surface area and there is essentially less, or no concern with respect to recombination of photo-generated carriers at defects on the perimeter of devices. Any such recombination has a negligible effect on the conversion efficiency, as the fraction of photo-generated carriers recombining at defects on the perimeter of the solar cell is small.
Another disadvantage is that the provision of sawing lanes on the semiconductor wafer, between solar cells, as well as the saw kerf, accounts for a sizeable portion of the wafer upon which the solar cells are formed. The material present in the sawing lane ultimately goes to waste instead of being used to convert light into electricity. This problem is exacerbated in small size solar cells, as the fraction of wasted material is higher when solar cells have a small surface area, i.e., when the ratio of cutting lane surface area to solar cell surface area is large. An additional disadvantage of sawing semiconductors wafers can generate residue that contains environmentally-unfriendly constituents. For example, in the case of III-V solar cells, that is, solar cells made of material comprising elements of groups III and V of the periodic table of the elements, the sawing process will generate residue that can contain arsenic, gallium, phosphorous, etc. The disposal of such residue is costly.
Improvements in solar cells are therefore desirable.